1. Field of the Invention
The present invention is related to heating and heaters for injection molding nozzles, more particularly for injection nozzles used in injection molding applications.
2. Background Art
Nozzles, nozzle arrays, micro nozzles and micro nozzle arrays have been used for many injection molding applications. An injection molding operation ideally maintains a constant or consistent viscosity and speed of a melt stream of moldable material through the one or more heated injection nozzles. When the viscosity and speed of the melt stream is maintained at a constant value, injection molded items that are uniform in appearance and have other desired characteristics can be produced. One way to control or maintain the viscosity and ideal melt stream speed is to have uniform heating along a nozzle channel in each injection nozzle used to form the item.
In an injection nozzle having a circular cross-section, a heating device can include a helical coil wrapped around a cylindrical nozzle body.
FIG. 13 shows a conventional nozzle 1302. In a nozzle 1302 having a non-circular cross-section, heating can be accomplished by inserting a heating rod 1304 alongside an entire length of nozzle channel 1308. This arrangement produces an almost ideal consistency in viscosity and melt stream speed. However, because heat transfer occurs along only one side of the nozzle channel 1308 and a small surface area of the rod 1304 is adjacent the nozzle channel 1308, there are still some areas of the melt stream that have varying viscosity and flow speeds. This is especially true when using small or tight pitch between nozzles 1302.
Some examples of flat nozzles are those manufactured by Mold Masters® Limited, Gunther Hotrunner Systems, and Heitec. An exemplary flat nozzle is found in U.S. Pat. No. 4,923,387 (“the '387 patent”), which shows an electrical heater plate connected to a nozzle. The outer shape of the nozzle is not defined. Another exemplary flat nozzle is found in U.S. Pat. No. 4,793,795 (“the '795 patent”) that shows a flat nozzle heated by a coiled heater, where the heater is embedded in the cylindrical surface of the flat nozzle. In the '387 and '795 patents, which are assigned to the assignee of the current invention, the heating element of the heater is a coiled wire that is a three dimensional (3D) structure occupying a rather significant space around or inside the nozzle. This makes the nozzles shown in the '387 and '795 patents somewhat bulky and impractical for inside gating and small pitch applications. Both of these documents are incorporated herein by reference in their entirety.
Small pitch nozzles having all flat lateral surfaces are shown in DE 19723374 (“the DE '374 document”) to Drach, which published on Dec. 18, 1997, and is assigned to Heitec Heisskanaltechnik GmbH, which is incorporated by reference herein in its entirety. In order to eliminate the impact of the heater on the nozzle size along one direction, the DE '374 document shows a nozzle having a rectangular body, a melt channel, and a tubular heater located along only one side of the melt channel. Placing the cylindrical coiled heater lateral with respect the melt channel increases the size of the nozzle along one direction and limits the size of the nozzle along the other direction.
A similar rectangular nozzle is shown in U.S. Published Patent Application No. 2002/0102322 A1 to Gunther (“the '322 PPA”), which published on Aug. 1, 2002, which is incorporated by reference herein in its entirety. The '322 PPA places the cylindrical heater along one side of the melt channel. Similar to the DE '374 document, the '322 PPA increases the size of the nozzle along one direction making it impractical for internal gating of small parts. This is because the DE '374 document and the '322 PPA provide cylindrical or 3D heating devices. These heating devices also provide a non-uniform heat profile along the nozzle melt channel.
The use of cylindrical 3D thin and thick film heaters for inside gating and small tight pitch applications may be achieved by the nozzle designs shown in U.S. Pat. No. 6,305,923 to Godwin et al. and U.S. Pat. No. 6,341,954 to Godwin et al., which are both assigned to Husky Injection Molding Systems Ltd. Similar round nozzles having 3D layered heater elements are disclosed in the U.S. Pat. No. 5,504,304 to Noguchi, U.S. Pat. No. 5,973,296 to Juliano, and WO 01/17317 to Gunther. All these patents are incorporated herein by reference in their entirety, and teach various 3D layered resistive heating elements, which may provide a compact design. Nevertheless making layered resistive heating elements on a cylindrical or 3D surface is on one hand a time consuming method and on the other hand is a method that cannot be applied to manufacture simultaneously a large number of heating elements in batches that deliver heaters having the same geometrical and functional characteristics. The use of a flat heating means permanently attached to a flat nozzle is taught by U.S. Published Patent Application 2003/0003188 A1 to Gunther (“the '188 PPA”), which is incorporated by reference herein in its entirety. However, the '188 PPA requires the heater device to be permanently coupled to the flat nozzle, which increases maintenance and replacement costs if the heater or nozzle were to fail.
Also, heaters in injection nozzles (e.g., injection molding nozzles) ideally produce a constant or consistent viscosity and speed of a melt stream of moldable material, which produces accurate items that are uniform in appearance. In conventional injection molding systems, when a heater starts to improperly function or fail completely, the entire nozzle might need to be removed from a manifold connection and replaced. Removing and replacing a nozzle requires shutting down a production line for an extended period of time. This increases manufacturing costs because of the expense of having to replace an entire nozzle, the inefficiency of production time, and the cost for labor being idle during removal and replacement of a nozzle.
Typically, injection molding nozzles have a heater element connected to one or more surfaces of a cylindrical or substantially flat or planar nozzle body. The heater element may be a tubular cartridge heater, film deposited, a clamped heater band, helical coil, or other type of heater. There are also injection nozzles including cast-in heaters, cartridge heaters, and heat pipe heaters located (embedded) entirely within the nozzle body proximate the melt channel. Embedded heaters tend to provide a desirable heat profile, a desirable heat transfer, and desirable heat efficiency because they are located in intimate contact within the nozzle body. Also each is disposed relatively closer to the melt channel than heaters connected to an outside surface of the nozzle body. There is a further class of flat nozzles incorporating cartridge heaters located either on one side or two, opposite sides of the melt channel. These flat nozzles are usually clustered into arrays of two or more nozzles and are utilized in areas with very limited space.
However, when certain film heaters are used, either thick or thin film heaters, they may not produce enough heat and/or may fail during processing. This can lead to defective products being produced and/or lost production time.
Therefore, what is needed is a system and method that allows a an injection nozzle heater to produce a desired amount of heat, while also compensating for failure of the heater so that lost production time is substantially reduced.